1. Field of the Invention
This invention relates generally to downhole tools, and more particularly to a downhole tractor for propelling working strings and wirelines in a wellbore.
2. Description of the Related Art
Subterranean operations in petroleum wells involve the conveyance of pipe, coiled tubing and wireline supported tools from the surface into well bores and vice versa. In vertical wells, and in those wells having only a few degrees of deviation, the axial thrust necessary to convey pipe or coiled tubing strings, or wireline tools, is supplied by gravity. In these situations, the downward thrust applied to the string is equal to the weight of the drill string, minus any buoyancy force due to fluid downhole. For pipe strings in relatively deep wells, this downward axial thrust can be quite formidable, sometimes exceeding 500 tons. Although the weight of a conventional coiled tubing string will be significantly less than a comparably sized drill pipe string, additional axial downward thrust is routinely applied to coiled tubing strings by a coiled tubing injector positioned at the surface.
The retrieval of pipe and coiled tubing strings, and wireline assemblies in vertical and slightly deviated wells is accomplished by applying upward axial thrust to the pipe string, coiled tubing string or wireline assembly as the case may be. In coiled tubing operations, this is routinely accomplished by reversing the direction of travel of the coiled tubing injector. In pipe strings, the pipe string is pulled from the well bore by platform mounted machinery. In wireline operations, though, the wireline conveyed tool or tool assembly is pulled from the well bore by retrieving the wireline or a cable that often is lowered into the well with the wireline assembly.
The situation is more complex in highly deviated and horizontal wells. In these types of wells, gravity can sometimes be relied upon to convey pipe and coiled tubing strings, and wireline assemblies into deviated sections, depending on factors such as the inclination of the well, the weight of the string and the magnitude of buoyant forces acting on the string. However, in most deviated well situations, the string will drag against the walls of the well bore at some point below the commencement of the deviated portion of the well. At this point, the string will not move downward further without the input of additional downward axial thrust. In pipe strings, additional downward thrust may be applied to the pipe string by means of surface equipment in order to advance the string through the deviated or horizontal section. The compressive load capacity of conventional pipe string is such that fairly significant levels of downward thrust may be applied without inelastically deforming or fracturing any of the pipe sections.
The relatively small outer diameters and wall thicknesses of coiled tubing place severe limits on the amount of surface-supplied downward thrust that can be applied to a coiled tubing string without buckling the tubing. Some surface supplied downward thrust is possible, and is usually imparted to the coiled tubing string via the coiled tubing injector.
As the skilled artisan will appreciate, a wireline itself is of little value in applying downward thrust to a wireline assembly. Other measures must be applied to deploy such downhole assemblies in highly deviated and horizontal wells.
Retrieval of pipe and coiled tubing strings, and wireline assemblies is also more complex in deviated and horizontal wells. During retrieval, the string or wireline assembly may bind against the inner walls of the well bore until the string is completely clear of the deviated section. As a consequence, an upward force exceeding the weight of the string or wireline assembly must commonly be applied during retrieval while the string or wireline assembly is within the deviated section. The capacity of the string or wireline assembly to withstand the overpull necessary to move such assemblies upward through a deviated well section is largely a function of the tensile strength of the string or wireline assembly. Conventional pipe strings can routinely withstand fairly significant tensile loads. Thus, their retrieval is largely a function of the power output of platform mounted retrieval machinery. Coiled tubing strings and wireline assemblies are more problematic in that their capacity to withstand tensile loads can be quite limited, particularly for wireline assemblies. If the tensile limit of a coiled tubing string or wireline assembly is exceeded, a costly fishing operation may be required to clear the wellbore.
Downhole propulsion machines, often referred to as "tractors", have been used for several years to facilitate the conveyance of wireline assemblies, and more recently, coiled tubing strings into a well bore. Most conventional tractors can be loosely grouped into two groups, namely, powered-wheel and crawlers. Most conventional wheeled-powered tractors consist of a tubular housing and two or more powered wheels that project from the housing and are designed to engage the inner walls of the casing, string or open hole, as the case may be, to propel the tractor and any portions of pipe or tubing or wireline tools connected thereto. Designers have developed several different types of wheeled tractor designs, some employing electrically powered wheels and some employing hydraulically powered wheels. In contrast, conventional crawlers typically consist of a housing and a reciprocating crawler mechanism that rhythmically engages and disengages the inner walls of the casing, string or open hole, as the case may be, to propel the tractor and any portions of pipe or tubing or wireline tools connected thereto.
Conventional wheeled tractors present certain shortcomings. One disadvantage common to many conventional designs is the lack of redundancy in power output to the propulsion wheels. In many conventional designs, a single power motor is encased within a tubular housing and coupled to multiple wheels by one or a plurality of mechanical linkages. These linkages typically consist of some form of complex shaft and U-joint arrangement with or without gearing, or a chain drive of some type. The difficulty with such designs is that power failure in the single drive motor results in loss of power to all of the drive wheels. Another disadvantage of such conventional designs is the sheer complexity of the mechanical leakages between the drive wheels and the common power motor. Such linkages routinely incorporate several cooperating sets of gears and shafts and/or chain sprockets that require relatively close tolerances in order to operate smoothly and without failure downhole. Another disadvantage common to some conventional tractor designs is the inability to provide axial movement in both directions. It is often desirable to be able to propel a string or wireline apparatus axially in both directions not only for insertion and retrieval purposes but also for downhole adjustment purposes. For example, logging operations routinely require several adjustments of the position of the logging tool relative to the bore hole prior to data acquisition. Such fine tuning of the position of the logging tool relative to the bore hole can be extremely difficult if the propulsion apparatus is limited to a single direction of travel.
Crawler type propulsion tools have the disadvantages of relatively slow travel speed and sometimes jerky longitudinal movements downhole. The relatively slow travel speeds of crawler type propulsion systems is a natural, though undesirable by-product of the reciprocating type of movement associated with the traction members of such devices. That very reciprocating type of movement also can lead to abrupt and jerky movements of the string downhole. Slow insertion and retrieval often translates into higher operating costs for the operator.
The present invention is directed to overcoming or reducing the effects of the one more of the foregoing disadvantages.